title: Few items in the thyroid-related quality of life instrument ThyPRO exhibited differential item functioning.
abstract: OBJECTIVE
  To evaluate the extent of differential item functioning (DIF) within the thyroid-specific quality of life patient-reported outcome measure, ThyPRO, according to sex, age, education and thyroid diagnosis.
  
  
  METHODS
  A total of 838 patients with benign thyroid diseases completed the ThyPRO questionnaire (84 five-point items, 13 scales). Uniform and nonuniform DIF were investigated using ordinal logistic regression, testing for both statistical significance and magnitude (∆R(2) > 0.02). Scale level was estimated by the sum score, after purification.
  
  
  RESULTS
  Twenty instances of DIF in 17 of the 84 items were found. Eight according to diagnosis, where the goiter scale was the one most affected, possibly due to differing perceptions in patients with auto-immune thyroid diseases compared to patients with simple goiter. Eight DIFs according to age were found, of which 5 were in positively worded items, which younger patients were more likely to endorse; one according to gender: women were more likely to report crying, and three according to educational level. The vast majority of DIF had only minor influence on the scale scores (0.1-2.3 points on the 0-100 scales), but two DIF corresponded to a difference of 4.6 and 9.8, respectively.
  
  
  CONCLUSIONS
  Ordinal logistic regression identified DIF in 17 of 84 items. The potential impact of this on the present scales was low, but items displaying DIF could be avoided when developing abbreviated scales, where the potential impact of DIF (due to fewer items) will be larger.
authors.affiliation: Department of Endocrinology, Copenhagen University Hospital, Rigshospitalet, Blegdamsvej 9, 2100, Copenhagen Ø, Denmark, torquil.watt@regionh.dk.
authors.forename: Torquil
authors.initials: T
authors.lastname: Watt
authors.forename: Mogens
authors.initials: M
authors.lastname: Groenvold
authors.forename: Laszlo
authors.initials: L
authors.lastname: Hegedüs
authors.forename: Steen Joop
authors.initials: SJ
authors.lastname: Bonnema
authors.forename: Åse Krogh
authors.initials: ÅK
authors.lastname: Rasmussen
authors.forename: Ulla
authors.initials: U
authors.lastname: Feldt-Rasmussen
authors.forename: Jakob Bue
authors.initials: JB
authors.lastname: Bjorner
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D015897:Comorbidity; D003718:Denmark; D004522:Educational Status; D005260:Female; D006305:Health Status Indicators; D006801:Humans; D016015:Logistic Models; D008297:Male; D001523:Mental Disorders / Q000150:complications / Q000175:diagnosis; D008875:Middle Aged; D011594:Psychometrics / Q000295:instrumentation*; D011788:Quality of Life; D057566:Self Report; D018434:Sickness Impact Profile; D011795:Surveys and Questionnaires; D013959:Thyroid Diseases / Q000145:classification / Q000150:complications / Q000175:diagnosis / Q000523:psychology*; D016896:Treatment Outcome; D055815:Young Adult